Provide ol.canvas instead of ol.canvas.SUPPORTED

This is not currently used outside the library, so it is not exported.

src/ol/canvas/canvas.exports

@@ -1 +0,0 @@
-@exportSymbol ol.canvas.SUPPORTED

src/ol/canvas/canvas.js

@@ -1,5 +1,4 @@
 goog.provide('ol.canvas');
-goog.provide('ol.canvas.SUPPORTED');
 
 goog.require('goog.dom');
 goog.require('goog.dom.TagName');